Indian doctors have found lead and nickel in the blood of patients who have succumbed to a mysterious illness in the southern part of the country.

More than 400 people in the state of Andhra Pradesh have fallen ill with common symptoms of nausea and fainting that does not appear to be linked to the coronavirus. At least one person has died.

The All India Institute of Medical Sciences (AIIMS) in the capital of Delhi tested 10 blood samples of patients and reported Tuesday they had found traces of lead and nickel.

The AIIMS did not find lead or nickel in water samples tested from the affected area.

Some officials in the state have said it is possible the illness is related to organochlorine pesticides, which are banned in many countries for their harmful health effects. It was not immediately clear how much, if at all, the pesticides are currently used in India.

Geeta Prasadini, public health director for the city of Eluru, said the patients first began exhibiting signs of the illness, including convulsions, Saturday night.

Commenting on the possibility that pesticides are to blame, Prasadini said Tuesday, “Nobody knows.”

Another Eluru official, Dolal Joshi Roy, told CNN Tuesday that none of the patients had tested positive for COVID-19, the diseases caused by the coronavirus.

Andhra Pradesh is one of the states hardest hit by the coronavirus, with more than 800,000 confirmed cases, Indian authorities said.

Roy said about 180 of those hospitalized for the illness had been released by Tuesday.